This invention relates to fatigue sensors and in particular to fatigue sensors which indicate a predetermined level of fatigue by undergoing a color change.
The lifetime of a structural part such as an aircraft panel or bridge girder is governed to a large extent by the stresses and strains to which the part is subjected and the environmental conditions to which the part is exposed. High stresses, deleterious environmental conditions, or combinations of both, will adversely effect a structural part so that eventually failure of the part will result.
There exists much information about failure of structural parts. For example, there is a great deal of empirical information relating to the effect of stress and enviromental conditions on structural parts made from various materials of construction including various metals, nonmetals and composite materials. Also, it is known that in most instances fracture of a part will be initiated at the surface of the material. Further it is generally known that both maximum stresses and strains and environmental factors play a major role in contributing to failure of structural parts and that these factors interact in a way that is not completely understood. The exact stresses to which a structural part will be subjected and the exact environmental conditions which will be encountered are very seldom known with certainty. Thus, the empirical data which is available and the generalized concepts which are presently known are not generally sufficient to indicate the probable remaining lifetime of a structural part.
Consequently, designers are now forced to employ unduly conservative safety factors in the design of such structural parts thereby resulting in greatly increased costs, uncertainty, and the inability to take advantage of the full available strength of the materials employed.
To reduce the amount of uncertainty that pervades the art of fatigue measurement, fatigue gauges have been developed to provide some measurement of the overload and fatigue history of structural parts as well as the probable remaining lifetime of structural parts. However, presently employed fatigue gauges do not adequately satisfy these goals. They generally comprise plastic encased constantan metal foil which undergoes a resistivity change under cyclic flexing. When attached to a structural member the gauges give an indication in the form of a resistivity change of the integrated cyclic strain which the member experiences. These prior art gauges have several drawbacks: complicated fabrication procedures must be used to insure that the gauges have uniform resistivity; temperature changes produce significant changes in resistivity thus producing unacceptable inaccuracies in fatigue measurements; and to obtain even qualitative fatigue measurements, it is necessary to monitor such gauges with an external calibrated resistivity apparatus.